Differential absorption LIDAR (DIAL) systems may be used to remotely measure the chemical composition of gases in the atmosphere. These gases may be present in the atmosphere or they may originate from other sources, such as gas leaks stemming from natural gas pipelines.
In an exemplary DIAL system, two lasers may be used. One laser may have a wavelength selected to coincide with a strong absorption feature of the gas to be detected. The other laser may have a wavelength selected in the wing (non-absorption region) of this absorption feature. The laser tuned to the absorption feature is referred to as the ON-line laser, and the other laser, tuned to the non-absorption region, is referred to as the OFF-line laser. Other exemplary DIAL systems may have more than one ON-line and/or OFF-line laser. For example, a DIAL system may have two (or more) ON-line lasers and one (or more) OFF-line laser. These systems, typically, may be mounted and operated from an airborne platform. These systems may also be mounted and operated from a ground-based platform.
For the two laser DIAL configuration, the DIAL system generates a composite signal containing two laser pulses in temporal sequence. One pulse is from one laser and the other pulse is from the other laser. The generated composite signal, which is a combination of the two laser pulses, is transmitted by a transmitter of the DIAL system to a target scene, where it reflects off the ground (or any other surface). Part of the reflected signal is detected and recorded by a receiver of the DIAL system.
If the gas of interest is not present in the scene, as shown in FIG. 7a, and the two pulses (one OFF-line and the other ON-line) are transmitted with equal strengths, as shown in FIG. 7b, then the intensities of the two pulses in the received signal are similar to each other, as shown in FIG. 7c. If the gas of interest is present in the scene, however, as shown in FIG. 7d, and the transmitted laser pulses have equal transmit intensities, as shown in FIG. 7e, then the intensities of the two pulses reaching the receiver are different, as shown in FIG. 7f. The intensity of the ON-line pulse is smaller than the intensity of the OFF-line pulse, because energy of the ON-line pulse is attenuated upon passing through the gas.
From this difference in intensity, an approximation to the concentration path length (CPL) of the gas may be estimated. A discussion of the concentration path length, as estimated by a DIAL system, is disclosed in U.S. Pat. No. 6,822,742, issued to Kalayeh et al., on Nov. 23, 2004, and is incorporated herein by reference in its entirety.
It will be appreciated that in many DIAL systems, the receiver captures samples of the reflected signal using an analog-to-digital (A/D) converter. The number of transmitted/collected samples is large (millions per second of dwell time). The OFF-line return pulse samples, typically, have higher return intensities or energies than the ON-line return pulse samples. Furthermore, the ON-line return pulse samples, typically, are not much higher in intensity or energy than the noise samples. This low signal/noise (S/N) ratio, when the gas of interest is present in the target scene, results in ambiguities or difficulties in detecting the ON-line pulse samples. These ambiguities and difficulties degrade the detection performance of the DIAL system.
The present invention addresses this problem by providing a method for locating the ON-line pulse samples and OFF-line pulse samples that are received by the DIAL system.